1. Field of the Invention
The present invention relates to a brushless direct current DC motor and more particularly to a brushless DC motor adapted for improvement in structure of a rotor to improve an efficiency and attenuate vibration and noise of the motor.
2. Description of the Prior Art
Generally, motors are designed to generate a high torque for obtainment of high efficiency and in order to generate a high torque, brushless DC motors are developed utilizing permanent magnets. In other words, a brushless DC motor utilizing a permanent magnet yields a higher efficiency than a motor without a permanent magnet due to a magnetic field formed by the permanent magnet.
FIG. 1 is a schematic diagram for illustrating a rotor of a brushless DC motor according to a first embodiment of the prior art, where the rotor 10 is disposed with a rotor core 12 on which a permanent magnet 14 is attached.
However, there is a problem in the first embodiment of the prior art in that the rotor 10 is rotated only by torque of the permanent magnet 14 according to repulse or suction of the permanent magnet 14, thereby being unable to maximize the efficiency.
FIG. 2 is a schematic diagram for illustrating a structure of a rotor in a brushless DC motor according to a second embodiment of the prior art, where the brushless DC motor includes a rotor 20 disposed in a stator 30 formed with a slot 32 therein between slits 26, where the rotor 20 is mounted with a permanent magnet 24 inside of the rotor core 22.
In other words, there is a problem in the brushless DC motor according to the second embodiment of the prior art thus constructed in that it rotates the rotor 20 by way of torque from the permanent magnet 24 and reluctance torque generated by coils of the stator 30 pulling the rotor core 22, such that it has a higher efficiency than the brushless DC motor according to the first embodiment of the prior art but creates more vibration and noise in inverse proportion to the improved efficiency.
The vibration and noise are generated by variation of electromagnetic torque, a kind of load and torque change of the load according to its characteristic, where the torque change is generated by saturation of the core, eccentricity and unsymmetrical structure of the rotor and the stator, uneven distribution of slit permeance caused by shapes of the stator slot and rotor magnetic poles, uneven states of the permanent magnet and windings and the like.
Particularly, changes of the axle which affects the greatest influence to a speed control characteristic of a brushless DC motor is largely generated by pulsation of slit flux density caused by an opening width of stator slit, shapes of the rotor magnetic poles and a mounted state of the permanent magnet. In other words, the brushless DC motor generates a mutual electronic suction force between the rotor core and the permanent magnet according to basic wave flux in a slit and harmonic flux, and the suction force is changed to generate vibration.
The harmonic flux components occurring in the slit further generate complicated electromagnetic harmonic components according to shapes of the rotor core and the stator core, and suction force or repulsive force is generated between stacked core sheet metals by the harmonic components thus described, thereby generating the vibration and noise.
Particularly, because a currently-used brushless DC motor for high speed and high output is a slot type brushless DC motor, cogging torque is generated by the slot and the vibration is further generated by the cogging torque.
At this time, the cogging torque acting as a noise source in the burshless DC motor thus described is generated by a difference of force of a magnetic substance and is applied because magnetic energy stored in the permanent magnet and slit between the stator teeth differs according to position thereof.
Furthermore, the period of the cogging torque is decided by the number of magnetic poles in a rotor and the combination of the stator teeth, where, a short period of the cogging torque corresponds to a reduction of intensity of cogging energy by way of dispersion of slit energy.
Accordingly, in order to reduce the cogging torque, a distance between the stator teeth should be adjusted but, however, there is a limit in adjusting the distance between the stator teeth. In other words, the stator teeth should maintain a mutually predetermined distance in order to insert windings, where the distance should be maintained by the winding method, such that there is a limitation in adjusting the distance between the stator teeth.
Consequently, flux density existing between a stator and a rotor should be enlarged and concurrently evened out in order to improve efficiency of a brushless DC motor and to reduce vibration and noise thereof, such that the shape of permanent magnet inserted into the rotor core is important.
In other words, magnetic permeability of a core is around 3,000.about.4,000(H/m), which is higher than that of air or a permanent magnet, and electromagnetic force induced from the stator generates a force for pulling a core of a rotor, such that it is very important to design a shape of a permanent magnet because the permanent magnet has a magnetic permeability almost identical to that of air to perform a capability of a slit.